The present invention relates to a fiber-optic gyro for use in detecting the posture of airplanes, space vehicles, and the like.
Known in the art is a fiber-optic gyro utilizing the Sagnac effect. In this gyro a laser beam from a laser diode is applied to a polarization plate through a beam splitter. Light having a specific polarization plane emerges from the polarization plate to strike an optical fiber directional coupler.
The coupling degree in the optical-fiber directional couplers, however, depends greatly on the polarization plane. The optical fiber directional coupler must therefore be designed in accordance with the polarization plate used so as to ensure the optimum coupling degree.
Even with a specially designed optical-fiber directional coupler, the optimum coupling degree may not be obtained due to the rotation of the polarization plane of the light due to terrestrial magnetism. To prevent this rotation, the light is converted to circularly polarized light by a depolarizer.
The polarization plate, depolarizer, and other related components increase the size and complexity of the gyro.
Also known in the art is a fiber optic gyro using a Rochon prism. A Rochon prism, however, is not always appropriate for branching and synthesis of polarized light between input and output optical fibers. To increase the efficiency of coupling between optical fibers, a special optical fiber therefore becomes necessary. Also, the end faces of the Rochon prism and the optical fibers are usually arranged perpendicularly to the optical path. Therefore, some incident light is reflected and returns to the light source to cause interference. Ordinarily, this return loss is about 14 dB per end face. Even if a non-reflective coating is applied, the return loss is only reduced to about 25 dB. Of course, the greater the number of the reflected faces in the optical path, the greater the return loss. Therefore, when utilizing a Rochon prism, it is impossible to control the return loss to a sufficiently low level.
The customarily used optical fiber has a circular section. Therefore, formation of different polarized components based on micro-bends or convexities and concavities on the boundary face of the core cannot be avoided. These different polarized components are produced by return light reflected on a reflecting face having a large return loss, resulting in cross-talk. In the conventional system, since the return loss is large, it is impossible to sufficiently reduce the cross-talk loss.
As a means for holding an optical fiber, there is known a method in which an optical fiber is held in a V-shaped groove extending in a certain direction, which is formed on a silicon substrate by etching. In the conventional system, since it is impossible to make both the output lights parallel to the input light, it is not permissible to utilize a V-shaped groove formed on a silicon substrate for holding the optical fiber. Therefore, the structure of the apparatus becomes complicated and increase of the size of the apparatus cannot be avoided.